Microperfusion is based on the exchange of substances across tiny holes of a small plastic tube called a "probe," which is inserted after numbing the skin over a fat pad. Through these openings, the fat pad is continuously rinsed with small amounts of fluid, and an exchange of substances occurs. The fluid drips from the other opening of the probe and is collected in a vial. Small amounts of study drugs are infused and the amount of substances released by the fat pad in response to them are measured.